1. Field of the Invention
The present invention relates to an optical amplifier used in an optical communication system, and more specifically, relates to an optical amplifier which has a polarization mode dispersion compensation function, to realize high functionality and also miniaturization.
2. Description of the Related Art
The optical amplifier is one of the key components for realizing long distance and large capacity in optical communication systems. The optical amplifier is classified into, a laser amplifier which uses stimulated emission from a population inversion medium, and an amplifier based on a nonlinear optical effect such as Raman scattering and Brillouin scattering. The laser amplifier includes a rare earth element doped optical fiber amplifier and a semiconductor laser amplifier using a semiconductor amplification medium. The former operates as an optical amplifier by photoexcitation, and the latter operates as an optical amplifier by injected current excitation.
Among these optical amplifiers, the rare earth element doped optical fiber amplifier has a large advantage in performance, such as being bit rate free, having high gain, low noise, a wide band, low coupling loss, low polarization dependency and high efficiency. Of the rare earth element doped optical fiber amplifiers, an erbium-doped fiber amplifier (EDFA) is most popular, and has been put to practical use in optical fiber communication systems.
Recently, the demand for communication traffic has increased tremendously due to the popularization of the Internet. As a technique responding to this need, wavelength division multiplexing (WDM) transmission technology, in which multi-wavelength optical signals are superimposed in one optical fiber to perform communication, is now attracting public attention. As for the WDM transmission technology, a point-to-point system mainly connecting two points has already been put to practical use. Moreover, demands for expansion of the wavelength region are further increasing, for strengthening the foundation for large capacity transmission, as Dense Wavelength Division Multiplexing (DWDM). By introducing the WDM transmission technology as described above, many optical amplifiers at least corresponding to the number of wavelengths of the signal light, become necessary for a one-wave optical amplifier in a transmitter/receiver. Therefore, demands for miniaturization of the optical amplifier are increasing.
Moreover, construction of a more flexible and more economical transport network is called for as a photonic network system exceeding the range of trunk long-distance and large-capacity systems, and development of optical node technology for constructing photonic networks is under way. Specifically, development of an optical add-drop multiplexer (OADM) having a function of dropping a signal of an arbitrary wavelength or adding a signal of an arbitrary wavelength from/to a WDM ring transmission line, and an optical cross connect (OXC) apparatus having a function of switching the optical lines mutually or a function as a wavelength switch for routing the light in units of wavelength, is now under way.
Further high function and diversification, as well as high speed, low cost and high reliability are required for photonic networks which are directed towards the upcoming large-capacity information oriented society. In order to respond to such needs, it becomes necessary to apply various kinds of compensation techniques in small units, such as in units of wavelength, or in groups of several wavelengths, to precisely control transmission quality. Specifically, it becomes necessary to apply functional devices such as an optical switch, a wavelength dispersion compensator, a polarization mode dispersion (PMD) compensator, a tunable optical filter and the like, to a required application area. Moreover, an optical amplifier corresponding to one wavelength or a group of several wavelengths is required in order to compensate for a loss occurring in such a functional device, and many optical amplifiers are necessary in one node. For example, it is expected that optical amplifiers in a level of several hundreds are required in one OXC apparatus.
Since it is assumed that the optical communication system described above is led in to a metropolis (urban network), and furthermore to close to offices and houses, it is expected that the demand for small optical amplifiers will increase further. Moreover, in higher speed optical communication systems, the need for wavelength dispersion compensators and PMD compensators increases. Particularly, it is necessary to arrange a PMD compensator for each signal channel. Therefore, it can be considered that many small optical amplifiers are required for compensating for losses in these optical parts.
An optical amplifier for compensating for such losses in functional devices is the one which has an amplification band corresponding to one wavelength or a group of several wavelengths, and is aimed at miniaturization while obtaining a gain sufficient for compensating for at least the losses of the required optical parts, differently from an optical amplifier which is aimed at high performance (for example, high gain, low NF, wide band, etc.) mainly developed heretofore.
As a conventional technique relating to optical amplifiers for compensating for losses, in Japanese Unexamined Patent Publication No. 5-502334, a single optical amplifier using a remote pumping method is installed on an input side of a single optical device (such as an optical coupler or an optical branching device), to integrate the single optical amplifier and the single optical device, thereby constituting an optical device with no loss.
Moreover, the present applicant has proposed in Japanese Unexamined Patent Publication Nos. 11-312848 and 2000-078081, an optical amplifier in which dispersion compensators (DCF) are arranged between stages of a plurality of optical amplifiers, so as to enable the compensation of wavelength dispersion with respect to wideband WDM signal light.
However, the conventional technique disclosed in Japanese Unexamined Patent Publication No. 5-502334 still remains in the configuration area in which the optical amplifier and the optical device are simply connected and combined, and space-saving of the optical amplifier and the optical device has not yet been realized. Moreover, in the technique proposed in Japanese Unexamined Patent Publication Nos. 11-312848 and 2000-078081, a dispersion compensator is arranged at an optimum position in the optical amplifier, taking into consideration the characteristics of the dispersion compensator and the optical amplifier. However, it has a configuration such that the optical amplifier and the optical device are simply connected and combined, as in the case of Japanese Unexamined Patent Publication No. 5-502334, and a problem of miniaturization of the optical amplifier has not yet solved.
The present applicant has proposed a technique for realizing space-saving, in which an optical device for compensating for losses is integrally provided on an output side of an optical amplification medium by using a connection method having an excellent reflection attenuation characteristic (such as a fusion splice method or a diagonal polished connector connection method), to thereby reduce a reflecting light on the output side to the optical amplification medium by utilizing an effect of insertion loss of the optical device, so that at least an isolator on the output side, of the isolators which have been heretofore necessary, can be omitted (see Japanese Patent Application No. 2002-092443). This prior application is for providing a technique relating to the configuration of the optical device for compensating for losses, and of the optical amplifier, but the type of the optical device is not particularly limited. Therefore, there is no technical approach to make common or share the required parts and space in order to realize the miniaturization of the optical amplifier, by specifying the optical device to be integrated with the optical amplifier.